Depends on how the GM handles the group, but if it is a group roll there is only a maximum +5. You need an quite perceptive adept/mage (say Int 4, Perception 5) actively heightened alert [IMO concentrating on only a small number of people] acting as the rep for the group get 17 dice (w/o shaman mask). Not that that shouldn't be enough, but it the situation can easly slip way.
Convert that into a club scence. Now you have mage/adept as an astral perceiving bouncer. It's his job so if it's something other than a dive he's likely something like Int 5, Perception 6. Someone with the caster would have to do something very stupid or look largely out of place to give the +3. Barring the no Etiquette, low Cha Uncouth troll tagging along that should be fairly rare.
![wink.gif](http://forums.dumpshock.com/html/emoticons/wink.gif)
So i'd give at least -2 distraction from all the people, and at least another -2 for interfering sensory (lots of different emotions rolling off that crowd). Unless they have another 5 Teamwork folks all astral perceiving they'll get another -2 for crappy light. So that's 5+6+2+2-2-2+5-2 = 14 dice for a solid pro bouncer team getting what i'd call a slightly generous ruling.
That's still looking ok to catch the Force 5 fool, and even Force 2 it looks like he might get away with it nearly 1/2 the time if he isn't careful about where he casts. But there are more metamagics to come, and i'll bet one of them will allow turning down visibility of casting. That is also assuming that the GM doesn't allow something like a Palming or Infiltration Opposed Test if the caster first identifies the likely spotters.